Low-noise corona discharge devices



Oct. 8, 1963 LOW-NOISE CORONA DISCHARGE DEVICES I Filed July 27, 1960 2 Sheets-Sheet 1 FIELD LINE FIG. I. v

TRAILING EDGE OF AIR FOIL v 8 DISTRIBUT CONDUCTING BODY RESISTAN l6 w :20 32 ROBERT WQEN ATTORNEYS FIG. 3. I

R. L. TANNER 3,106,663

Oct. 8, 1963 R. L. TANNER LOW-NOISE CORONA DISCHARGE DEVICES Filed July 27, 1960 2 Sheets-Sheet 2 38 FIG. 5A.

82 REGION OF VORTEX W FORMATION FIG. 6.

INVENTOR. ROBERT L. TANNER D. C. ELECTRIC FIELD LINES ATTORNEYS United States Patent Radio receivers, which are transported in vehicles such as aircraft, are subjected to noise arising from signals produced in the receiver antenna as a result of corona or static discharges which occur at various portions of the vehicle. The modern aircraft usually carries some form of static discharger placed at what are thought to be strategic locations in order to minimize the pickup caused by such discharges, but there is still considerable noise present.

An object of this invention is to provide an arrangement for minimizing the noise caused in a radio receiver carried by a vehicle, in response to corona discharges from said vehicle.

Another object of this invention is to provide a novel and improved static-discharge noise-minimization arrangement.

Still another object of the present invention is the provision of an improved arrangement for causing a corona discharge on a vehicle to occur at locations and in directions which minimize deleterious effects which can occur due to corona discharge.

Yet another object of the present invention is the provision of an improved arrangement for causing a corona discharge on a vehicle to occur at locations and in directions which minimize deleterious effects which can occur due to static potentials.

These and other objects of the invention may be achieved by causing corona discharges to occur at strategic locations about a vehicle, such as an aircraft, in a manner so that current flow characterizing the corona discharge is substantially at right angles to the direction of the field which would be established were the receiver antenna excited with a voltage, in locations where the said receiver antenna field has a minimal or substantially null value. Further, in accordance with this invention, corona dischargers may have a bias applied to them in order to generate corona discharges regardless of the static potential on the vehicle, whereby the moving vehicle is prevented from being charged up to voltage levels of sufiicient magnitude to cause hazardous discharges of said potential to occur.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a drawing representing the field configuration of a radio-frequency antenna field produced about a conducting body connected to an air frame by a distributed resistance;

FIGURE 2 is an enlarged view of the low-field region represented in FIGURE 1, as well as the placement of a corona discharge pin relative thereto in accordance with this invention;

FIGURE 3 shows another arrangement for corona pin placement in accordance with this invention;

FIGURE 4 illustrates placement of the embodiment of the invention on an aircraft, for example;

FIGURES 5 and 5A illustrate an arrangement for biasing a corona discharger in accordance with this invention;

FIGURE 6 is a view of an air foil tip, shown to illustrate a region of vortex formation;

FIGURE 7 illustrates how the embodiment of the invention may be employed with the air foil tip of FIG- URE 6 to eliminate difliculties arising with vortex regions;

FIGURE 8 is a section along the lines .8-8 of FIG- URE 6, illustrating the DC. electric field lines; and

FIGURE 9 illustrates how the effects of these D.C. electric field lines are minimized by employing an embodiment of the invention.

In Patent No. 2,933,732, by this inventor, there is shown how the radio-frequency noise produced in radio systems by corona discharges can be reduced by causing the discharges to occur at points of minimum antenna field. The invention described in that patent is based on a principle of electromagnetic theory, which states that the noise current produced at the terminals of an antenna by an electrical discharge occurring at some point in the vicinity of the antenna is given by the following relationship:

I 1 ii .7

tn 7af a a The integral is taken throughout the volume in which density vector I It is important to note that I is a vector quantity; that is, it is a quantity defined in the region of space over which the integral is taken, having both magnitude and direction. The quantity E, is the electric field that would be produced in the vicinity of the antenna by the voltage V applied to the antenna terminals. E is also a vector quantity. :The relationship given is a reciprocity relationship, of a type well known in the study of electromagnetic fields. It should also be noted that the two situations employed, that is, a terminal noise current existing in response to an electric discharge occurring in the vicinity of the antenna, and an electrical field existing in the vicinity of the antenna in response to a voltage applied to the antenna terminals, are not assumed to exist simultaneously. The second situation, in fact, need not actually occur at all. It is simply the condition that Would exist if a voltage were applied to the antenna terminals. Hereafter where radiofrequency fields are referred to, it will be understood that it is these hypothetical reciprocal fields which are meant.

In the invention described in Patent No. 2,933,732, distributed resistances, combined with conductors, are employed to produce localized modifications of the hypothetical radio-frequency fields of the antenna on an aircraft such that minirna regions of these fields exist. The invention then goes on to teach that the noise-producing discharges can be caused to occur from sharpened pins located in these minima, thereby resulting in reduced noise currents at the antenna terminals with consequent reduced noise in the receivers connected to these terminals.

In order to appreciate the novel features of the present invention, it should be noted that the quantities E,,-],,, which occur in the equation given previously, represent the so-called dot product of two vector quantities. Since these quantities are vectors, they are characterized by both direction and magnitude, and the operation represented by their dot product is the product of the magnitude of the two quantities multiplied by the cosine of the angle between the lines characterizing their directions.

In symbols,

E -Jn=lE,.[ ]J,,l cos 0 where 1E,,l and U i are the magnitudes of the two vector quantities and 0 and the angle between them.

In the Patent No. 2,933,732, it is taught that noise may be reduced at the antenna by so manipulating the field E that its magnitude [BI is minimum in the vicinity As noted previously, the quantity E,,-],,, which governs the magnitude of the noise produced, depends upon the quantities E and I as well as By suitable manipulation, the

angles characterizing the upon their magnitudes.

field E, is made to have a direction at right angles, or

In this way,

the factor cosine 0 that occurs in the dot products E,,-J is made to have a small value even though the magnitudes of the quantities are appreciable.

Consider, for example, the discharger of the general type described in Patent No. 2,933,732, which is represented by the drawing in FIGURE 1. To the trailing edge of an air foil 19, which can be a wing-tip, tail-tip, or the like, there is attached by means of a distributed resistance 14 a body 12. The distributed resistance may be fabricated of a high resistance material or an insulating material with a high resistance coating. Also as shown in the patent, plane surfaces may also be used. The body 12 may be conductive or have a conductive surface. The radio-frequency field is represented by the field lines 16. The conducting body 12, being connected to the airframe only by the high distributed resistance, appears isolated from the airframe at radio-frequency. It is shown that the presence of the body in the field modifies the field in such a manner that regions are produced about the body in which the field is low. These regions are represented by the crosshatched areas 18 adjacent the conducting body 12. Discharge points which are placed in these regions will minimize the coupling to the antennas on the aircraft.

FIGURE 2 shows an enlarged view of one of these regions of low field with the placement of a discharge point 20 therein in accordance with this invention. Although the radio-frequency field is small in the regions 18, there is still suificient residual value, as represented by the lines 17, so that there is still some coupling resulting when a discharge occurs in this region due to corona, with consequent noise developed in the radio receiver. However, in accordance with this invention, the discharge pin 20 concentrates the DC. field and tends to concentrate the corona discharges at its tip. The components of discharge current fiow in these discharges responsible for the production of radiofrequency interference is confined to substantially right angles, to the vector I a small region in space, as represented by the lines 22. Further, the direction of flow of these current components tends to be coincident with the axis of the pin 20. Thus, for a pin located anywhere, and with any orientation in the region of low-coupling fields, coupling of the discharges is small, and noise experienced in the antennas will be low. However, if the pin is positioned and oriented as shown, so that the average direction of the lines of current flow is at right angles to the direction of the RF field lines in the region of the discharge, the coupling of noise from corona is still further reduced.

Another embodiment of the invention may be seen in FIGURE 3, wherein a dielectric rod 30 which may be made of a plastic such as nylon, is coated with a high resistanee paint 32 and is attached to an aircraft at the tip of an air foil service 34, or other extremity, where by the DO field, which tends to induce the corona discharges, and the hypothetical RF field, which is a measure of the strength of the coupling to the antenna, tend to concentrate. Discharge pins 36, 38 may be mounted on the dielectric rod in accordance with the teachings of this invention. There need be no metal in the rod except for the discharge pins. Alternatively, the rod may be made homogeneously resistive by dispersing carbon in the plastic material. The coating of a dielectric rod with a resistive coating is preferred, however.

he presence of the resistively coated rod modifies the configuration of the RF field in its vicinity. The component of the RF field which is radial to the axis of the rod experiences a minimum at a distance from the outer end which depends on the resistivity and to some extent on the frequency of the field. Using the wellknown field-intensity measuring techniques, or the mathematics of field distribution, a point can be readily found at which the radial field is very small over a wide frequency range. At this point there exists a small but significant axial component of the field. The DC. field inducing the corona discharge is such that if discharge pins are mounted in the rod with a radial orientation, the flow of current in the discharge tends to be radial or at right angles to the existing axial RF field, thereby minimizing any noise. Effectively, the field conditions achieved by the structure shown in FIGURE 3 are substantially represented by the field lines in FIGURE 2.

FIGURE 4 is a schematic representation of typical locations of the embodiment of the invention on an aircraft. These should be considered as exemplary, and by no means limiting, neither with respect to the locations shown nor the numbers of discharges shown. The aircraft 40 has three receivers 42, 44, 46 (represented by rectangles), which are shown positioned at the forward end of the aircraft 4%. One of these 42 is the receiver for a flush-mounted automatic direction finder which has a sense antenna 43 recesssed in the belly of the aircraft and a loop antenna (not shown). The second and third receivers 44, 46 are connected to their antennas (respectively represented by rectangles 5t 52), which may be located under a plastic cover on the tip of the vertical stabilizer 54. The antenna 50 represents one which is employed as the loran antenna, and the antenna 52 may be employed as the high-frequency receiving antenna. The locations of the embodiments of the invention are represented by the groups of lines 56, 58, 60, 62, which extend from the outer tips of the wings, the outer tips of the horizontal stabilizers, and the top of the vertical stabilizer.

Construction of the dischargers embodying this invention is such that when they are located at the extremities of aircraft or other vehicles and structures, at which concentrations of DC. field tend to exist, particularly strong concentrations of field occur at the tips of the discharge points. Corona discharges therefore tend to occur at these decoupled discharger points in preference to other points on the vehicle or structure from which coupling to the antennas is high. In order for the necessary high DC. fields to exist at the discharge points, however, it is usually necessary that the vehicle to which these discharge points are attached, if such vehicle is isolated from ground, as is the case of an aircraft in flight, have substantial DC potential with respect to ground.

Under most circumstances, this vehicle potential causes no adverse eifects other than the corona discharges which have been described. There are situations, however, in

which such a potential is definitely undesirable. For example, in the case of a hovering helicopter which may be charged to high voltages by the impingement of dust particles on the rotor blades. Sparks produced by these voltages when helicopters pick up grounded cargo or personnel can be hazardous if the cargo is inflammable or explosive, and can be dangerous to personnel. In the case of other types of aircraft which may be used, for example, in carrying instruments to measure ambient, atrnospheric electrical fields, the static field arising as a result of the precipitation charge, or, in the case of jet aircraft, as the result of charging caused by the jet engine, can prevent accurate measurements of the ambient electrical field.

To dispel such charges, which accumulate on the vehicle, without the production of radio noise and without the necessity of having the vehicle assume a high voltage, in accordance with this invention an arrangement may be employed such as shown in FIGURE 5. As before, this will involve positioning discharge points in the RF field minima with an orientation so that the discharge current flow is at substantially right angles to the RF field. In addition, a bias voltage is applied to enable discharges to occur at lower vehicle voltages.

Referring now to FIGURE 5, there is seen in section an airfoil trailing edge 64, or other suitable portion of the vehicle structure. Carried therein is a high-voltage direct-current supply 66, one terminal of which is grounded to the vehicle structure by means of a strap 68. The high-potential side of the high-voltage supply 66 is connected to the distributed resistance supporting structure 68 of the type described in connection with FIGURE 1, with the difference that the distributed resistance is insulated from the airframe. The distributed resistor is also connected to the vehicle structure by means of a bypass capacitor 70. Thus, the high voltage supply 66 is connected between the distributed nesis-tance and the body of the aicraft.

The distributed resistance, which may be one of the structures previously mentioned, is connected to a conductive body 72, on which there are discharge points 74, 76. These are oriented in accordance with the previous teachings for the purpose of establishing the discharge current flow lines at right angles to the lines of the RF field in a minimum RF field region. The structure of the distributed resistance 68, conducting body 72, and pins 74, 76 may be replaced, if desired, by the structure shown in FIGURE 3 of a plastic rod coated with high-resistance paint and a plurality of discharge pins. This is exemplified in FIGURE 5A. Note that here the resistive coating 3% does not contact the wing surfaces, but stops short thereof. It is connected to the surfaces through the potential supply 66-;

Because of the application of the high-voltage supply, there will be concentrations of D.C. field at the tips of the discharge pins, even though the vehicle as a whole has no voltage. This field causes discharges to occur at the tips of the discharge pins, resulting in the formation of ions in the space immediately surrounding the tips of the pins. If the voltage applied to the discharger assembly is positive, these will be positive ions, and if the voltage is negative, they will be negative ions. These ions are swept away from the vehicle by the slip stream, resulting in the removal of charge of one sign or another from the vehicle.

It should be noted that the ability to apply voltage of either sign to the discharging assembly creates a D.C. field at the discharge points which may either augment or override any field existing there due to a voltage on the vehicle as a whole. Thereby, the device described may function either as a charger or as a discharger. Its charging and discharging capacity is approximately pro portional to the voltage applied, and also to the spatial extent of the structure in a direction transverse to the slip stream. The arrangement shown in FIGURE 5 may be employed at any one or more of the locations shown in FIGURE 4 for dischargers on an aircraft, or in other locations which may be found desirable.

A still further improvement in accordance with this invention, in eliminating noise created in receivers as a result of corona discharges, can be effectuated by locating dischargers in accordance with this invention in a manner to be described. As described above, the usual location of these dischargers is at the extremities of an aircraft or other vehicle, where the dischargers can go into corona when relatively low voltages exist on the vehicle as a whole. This is efiectuated because of the great concentration of field occasioned by the extremely sharp points of the projecting discharge pins. Where an aircraft is charged up by precipitation impingement, the amount of current carried by a particular discharger increases approximately in proportion to the voltage on the aircraft. As the amount of charging increases, therefone, the voltage on the aircraft must also increase in order that the current being discharged equals the charging current. Although the current leaving the aircraft by Way of the low-noise discharge devices results in a negligible noise in the receivers, if the charging rate becomes high enough, the aircraft voltage will become so high that discharges will occur at points on the aircraft other than the points provided by the discharger devices and from which their effects will couple strongly into the antennas.

In aircraft, points at which such discharges are likely to occur are the regions of vortex formation at the tips of air-foil surfaces. One characteristic of such vontices is a large reduction in the density of the air at their cores. It is well known that a reduction in the density of air causes a corresponding reduction in the strength of the electrical field necessary to cause corona. For this reason corona discharges can occur at the air-foil tips in the vortex regions at aircraft voltages much lower than might be expected on the basis of relatively large radii of curvature of the structure in these regions.

To guard against the occurrence of discharges arising as a result of the vortex phenomena, use is made of the field reducing or shielding effect of a space charge produced by discharges from low-noise decoupled dischargers. This efiect is illustrated in the sequence of FIGdURES 6, 7, 8, and 9, to which reference will now be ma e.

FIGURE 6 shows a wing or air-foil tip 80, wherein the region of vortex formation is indicated by the shaded area 82. FIGURE 7 represents a series of decoupled dischargers 84, 86, 88 in accordance with this invention, which are attached to the tip at the region of vortex formation. These dischargers may be of the type shown in FIGURE 3. They contain discharge pins 84, 86, 88. The threshold voltages of these dischargers is relatively low. When they discharge or go into corona, they form an ionic space-charge cloud which is swept back by the slip stream and urged outward from the air-foil tip by the existing field. The configuration of the spacecharge cloud which is due to current leaving the decoupled dischargers 84, 86, 88 is represented bv the dashed lines 90 in FIGURE 7.

Referring now to FIGURE 8, there may be seen a section taken along the lines 8+8 of the air-foil tip 80. This section 92 has extending therefrom lines 94, which represent the D.C. electric field lines Without the use of this invention. From the configuration of the D.C. electric field indicated by these lines 9'4, it will be seen that the intensity of the field at the surface of the air-foil tip is proportional to the density of the lines.

FIGURE 9 shows the air-foil section 92 with a cloud of space charge arising from current leaving the decoupled dis-chargers in accordance with this invention. The space charges or ions are represented by the small circles 96 with the negative sign therein. As shown, the field actually existing at the surface of the air-foil tip is much less in the presence of space charges than in their absence. The field lines 94 terminate on the space charges. 96 rather than on the metal surface. Thus, the possibility" of corona occurring due to the combination of a vortex low-pressure region and DC. field is considerably diminished, if not eliminated.

As previously pointed out, the location of the decoupled dischargers in accordance with this invention is determined for the particular type of vehicle by its air-foil trailing edges, at which a discharge is most likely to occur. Decoupling dischargers should also be placed in those regions where vortex formation is most likely to occur. This can be determined from wind-tunnel studies or from knowledge of the characteristics of the air-foil surfaces of an aircraft. Further, in accordance with this invention, Where it desired to continuously drain any charges from the body of the vehicle, a bias may be applied to a discharger in accordance with this invention generate corona discharges which drain off any static charges which would otherwise accumulate on the body of the vehicle.

Accordingly, there has been described and shown herein a novel, useful arrangement for minimizing, far beyond what has been heretofore achieved, noise detected by receivers on moving vcmcles as a result of corona discharges.

I claim:

1. In a system of the type wherein noise from corona discharges are detected at the terminals of an antenna, an improved noise-minimizing arrangement comprising a corona discharge element shaped for establishing predetermined discharge-current flow lines, and means for supporting said means for discharging corona at a minimum region within the radio frequency coupling field of the antenna yet still within the direct-current field with said discharge-current flow lines substantially at right angles to the direction of the residum radiof-requency coupling field of said antenna at said minimum region.

2. In a system of the type wherein a vehicle carries a receiver having an antenna, said vehicle having surfaces from which corona discharges occur and there are a plurality of corona discharge pins having discharge points supported from said vehicle surfaces to discharge corona in the region of minimum regions of the radiofrequency coupling field of said antenna, the improvement comprising means for supporting each of said plurality of discharge pins with the axis extending through its point substantially at right angles to the direction of said antenna radiofrequency coupling eld within said minimum region.

3. A corona discharge noise-decoupling device for a structure carrying a receiver having an antenna, said structure having surfaces at which corona discharges occur comprising means connected to one of said surfaces for establishing a minimum radiofrequency coupling-field region for said antenna, a corona discharge pin, and means for mounting said corona discharge pin on said means for establishing a minimum radiofrequency cou ling-field 8 region, within said region for providing a discharge-current path substantially at right angles to the radiofrequency field within said minimum region.

4. A corona discharge noise-decoupling device as recited in claim 3 wherein there is included a source of discharge potential supported by said structure, and means for connecting said source of discharge potential between said structure and said corona discharge pin to establish corona discharge to thereby prevent an accumulation of electric charge by said structure.

5. A corona discharge noise-decoupling device for a structure carrying a receiver having an antenna, said structure having surfaces at which corona discharges occur comprising means connected to one of said surfaces for establishing a minimum radiofrequency coupling-field region for said antenna, sm'd means including a resistive rod-like member extending away from said one of said surfaces, a corona discharge pin, and means for mounting said corona discharge pin on said resistive rod-like member Within said minimum radiofrequency couplingfield region for providing a discharge-current path substantially at right angles to the radiofrequency field within said minimum region.

6. In a vehicle of the type carrying a receiver having an antenna, said vehicle having surfaces which accumulate static potential, apparatus for discharging said accumulated static potential without increasing noise in said receiver comprising means connected to one of said surfaces for establishing a minimum radiofrequency coupling-field region, said means including a body having conductive surfaces, and an elongated resistive means having one end insulatingly attached to said one of said surfaces and the other end attached to said body to hold it spaced from said surface, a corona discharge pin, means for positioning said corona discharge pin on said body conductive surface within said minimum radiofrequency coupling-field region for providing a discharge-current path substantially at right angles to the radio-frequency field within said minimum region, a source of discharge potential, and means connecting said source of discharge potential between said one of said surfaces and said resistive means to establish corona discharge at said pin to thereby prevent an accumulation of electric charge by said structure.

References Cited in the file of this patent UNITED STATES PATENTS 2,309,584 George Ian. 26, 1943 2,333,975 Bennett Nov. 9, 1943 2,397,ll8 Bennett Mar. 26, i946 2,536,818 Lawton Jan. 2, 1951 2,933,732 Tanner Apr. 19, 1960 FOREIGN PATENTS 690,379 Great Britain Apr. 22, 1953 

1. IN A SYSTEM OF THE TYPE WHEREIN NOISE FROM CORONA DISCHARGES ARE DETACTED AT THE TERMINALS OF AN ANTENNA, AN IMPROVED NOISE-MINIMIZING ARRANGEMENT COMPRISING A CORONA DISCHARGE ELEMENT SHAPED FOR ESTABLISHING PREDETERMINED DISCHARGE-CURRENT FLOW LINES, AND MEANS FOR SUPPORTING SAID MEANS FOR DISCHARGING CORONA AT A MINIMUM REGION WITHIN THE RADIO FREQUENCY COUPLING FIELD OF THE ANTENNA YET STILL WITHIN THE DIRECT-CURRENT FIELD WITH SAID DISCHARGE-CURRENT FLOW LINES SUBSTANTIALLY AT RIGHT ANGLES TO THE DIRECTION OF THE RESIDUAL RADIOFREQUENCY COUPLING FIELD OF SAID ANTENNA AT SAID MINIMUM REGION. 